A pipe bending mandrel that is expandable is disclosed in U.S. Pat. No. 3,109,177 to Avera et al. The mandrel disclosed in the Avera patent comprises a plurality of longitudinally spaced expansion units for radial expansion, each unit having a ring assembly secured to the mandrel frame so that a plurality of radially reciprocal rams extend to a single locked position for contacting the pipe along an interior circumference.
U.S. Pat. No. 3,851,519 to Clavin et al. discloses an internal pipe supporting mandrel having upper and lower assemblies that are movable away from and toward each other for expanding and collapsing the mandrel. The upper and lower assemblies are moved apart by transverse movement of bosses mounted between the upper and lower assemblies against cam surfaces secured to the upper and lower assemblies thereby moving apart the upper and lower assemblies. When the mandrel is fully expanded, the bosses are in engagement with flat shoulders on the upper and lower assemblies to lock the mandrel in a single expanded position which also represents the maximum distance that the upper and lower assemblies may be separated.
U.S. Pat. No. 4,086,803 to Wheeler discloses an internal pipe supporting mandrel that expands radially to contact the interior of a pipe in which a plurality of segments are connected together and expand radially by movement of spherical members over tapered sections of the segments.
A need exists for an economical, flexible, vertically expandable mandrel of simple design and construction that expands by longitudinal movement in which the mandrel is capable of applying a predetermined variable supporting force to top and bottom interior cylindrical segments of pipe sections despite variations in size from section to section and despite wear of the mandrel that may occur from repeated use.